Method of cooling cold steel strip with cooling rolls

ABSTRACT

A method of cooling band or hoop steels with hollow cooling rolls through which a cooling medium flows to absorb the heat of the steel strips trained about and in contact with the hollow cooling rolls, thereby cooling the steel strips. According to the invention this method is carried out so as to fulfil the following equation (1) with the cooling roll having a diameter D&gt;600 mm when a thickness h of the steel strip is within 0.2≦h&lt;0.6 mm, 
     
         ΔT.sub.S &lt;0.65·σ.sub.T.sup.1.5 
    
      ·θ·h -0 .75                    (1) 
     and so as to fulfil the following equation (2) with the cooling roll having a diameter D&gt;1,000 mm when a thickness h of the steel strip is within 0.6 mm≦h, 
     
         ΔT.sub.S &lt;1.05·σ.sub.T.sup.1.5 
    
      ·θ·h -0 .83                    (2) 
     where ΔT S  is temperature fall °C. per one cooling roll, σ T  is tensile stress in a lengthwise direction of the steel strip and θ is winding angle about the cooling roll.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a roll cooling method for cooling cold steelstrips, and more particularly to a method of cooling steel stripe withrolls in a heat treatment line, particularly continuous annealingprocess line or continuous plating line.

2. Description of the Prior Art

It has been generally known to continuously cool a cold steel strip incontact with and running about a hollow roll with the aid of heattransfer between the steel strips and a cooling medium flowing throughan inner cavity of the roll. However, the steel strip cooled by such aroll cooling method often does not keep its flatness after cooled andtends to cause defects such as wave-like deformations, wrinkles or foldswhich inadmissibly reduce its value in article of commerce.

There are two factors making defective the shape or appearance of thesteel strip. One relates to an accuracy of an apparatus, such asdeviated shapes of cooling roll surfaces, dirty surfaces of the coolingrolls, incorrect setting of the cooling rolls and the like. The otherrelates to an operating condition such as unsuitable selections ofcooling roll diameters, lengthwise tensile forces acting upon steelstrips, cooling extent for steel strips, winding angles of the steelstrips which are central angles at centers of the rolls subtended byparts of the steel strips wound about the rolls, and the like.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved method ofcooling steel strips with cooling rolls, which limits the aboveoperating conditions within predetermined ranges to remove the abovesource causing shapes of the steel strips to be defective, therebykeeping the flatness of the steel strips after cooled.

In order to achieve the above object, according to the invention themethod of cooling a steel strip with a hollow cooling roll by means ofthermal transmission through the roll between a cooling medium flowingthrough an internal cavity of said cooling roll and said steel stripsbeing in contact with and running about said cooling roll is carried outso as to fulfil the following equation (1) with said cooling roll havinga diameter D>600 mm when a thickness h of said steel strip is within0.2≦h<0.6 mm,

    ΔT.sub.S <0.65·σ.sub.T.sup.1.5 ·θ·h.sup.-0.75                    ( 1)

and so as to fulfil the following equation (2) with said cooling rollhaving a diameter D>1,000 mm when a thickness h of said steel strip iswithin 0.6 mm≦h,

    ΔT.sub.S <1.05·σ.sub.T.sup.1.5 ·θ·h.sup.-0.83                    ( 2)

where ΔT_(S) is temperature fall °C. per one cooling roll, σ_(T) istensile stress in a lengthwise direction of said steel strip and θ iswinding angle about said cooling roll.

In order that the invention may more clearly understood, preferredembodiments will be described, by way of example, with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a steel band being cooled by ahollow cooling roll partially removed;

FIG. 2 shows a temperature distribution in a traverse direction of asteel strip being cooled by a roll;

FIG. 3 illustrates a stress distribution in the traverse direction ofthe steel strip shown in FIG. 2;

FIG. 4 is a perspective view of a steel strip which is defectivelydeformed due to the stresses;

FIG. 5 is an explanatory view of a winding angle θ₁ and a contact angleθ₂ of a steel strip about a cooling roll;

FIG. 6 is a schematic perspective view of a contact angle distributionof a steel strip about a cooling roll;

FIG. 7 is a graph illustrating a relation between temperature fallsΔT_(S) per one roll and average winding angles θ of steel strips havinga 0.4 mm thickness about rolls having 600 mm diameters;

FIG. 8 is a graph similar to FIG. 7 but with steel strips having a 1.0mm thickness and rolls having 1,000 mm diameters;

FIG. 9 is a graph illustrating an adoptable tensile stress range inlongitudinal direction of steel strips having thicknesses 0.2≦h<0.6 mmwound about rolls having 1,000 mm diameters;

FIG. 10 is a graph similar to FIG. 9 but with steel strips havingthicknesses 0.6≦h≦2.3 mm and rolls having 1,200 mm diameters;

FIG. 11 is a graph illustrating relations between temperature fallsΔT_(S) per one roll and tensile stresses σ_(T), with steel strips havinga 0.4 mm thickness wound with average winding angles θ=30°, 60°, 90° and120°;

FIG. 12 is a graph showing relations between temperature falls ΔT_(S)per one roll and thickness h of steel strips wound thereabout withaverage winding angles θ=30°, 60°, 90° and 120° and subjected to 1kg/mm² tensile stresses;

FIG. 13 is a graph similar to FIG. 11 but with steel strips having a 1mm thickness; and

FIG. 14 is a graph similar to FIG. 12 but steel strips thicker thanthose in FIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In general, when a cold steel strip is wound about a cooling roll, acentral angle at a center of the roll subtended by a part of the steelstrip actually in contact with the roll is different from a centralangle at the center of the roll subtended by a part of the steelintended to wind about the roll depending upon a rigidity of the steelstrip because of a tendency of the steel strip to become straight. Inthis specification and claims, a central angle at a center of the rollsubtended by a part of the steel strip actually in contact with the rollis referred to as "contact" angle, and a central angle at a center ofthe roll subtended by a part of the steel intended to wind about theroll is referred to as "winding" angle which is a theoretical orgeometrical angle. It has been found that a flatness of a steel strip isaffected by a temperature distribution on the steel strip in itstraverse or lateral direction, which is in turn dependent upon thecontact angle and cooling action of the roll.

The inventors of this application have further investigated the factorconcerning the operating condition which makes defective the shape orappearance of a steel strip after cooled, in cooling by a cooling medium3 flowing as shown by an arrow through a cavity of a hollow cooling roll2 about which a steel strip 1 is trained. As the result, the followingmatters have been found.

The defective deformation of the steel strip is fundamentally due to thefact that a temperature distribution on the steel band 1 in its traversedirection is uneven as shown in FIG. 2 to cause a stress distribution inits longitudinal direction as shown in FIG. 3. In other words,compressive stresses occur in the part of the steel strip where thetemperature is relatively high as shown in FIG. 3. When the compressivestresses exceed a determined value, the steel strip can no longer keepits flatness to cause a buckling resulting in a deformed steel strip asshown in FIG. 4.

The temperature difference in the traverse direction of the steel stripis caused by the fact that when a steel strip 1 is wound about a coolingroll 2, a contact angle θ₂ is generally smaller than a winding angle θ₁which is geometrical. A reference numeral 5 in FIG. 5 denotes tangentiallines to a circle of the roll 2. The winding and contact angles θ₁ andθ₂ have the following relations.

(1) When tensile forces in the longitudinal direction of the steel stripare increased, the contact angle θ₂ approaches the winding angle θ₁.

(2) As a diameter of the cooling roll increases, the contact angle θ₂approaches the winding angle θ₁.

(3) As a thickness of the steel strip decreases, the contact angle θ₂approaches the winding angle θ₁.

The above relations (1), (2) and (3) are indicated as an equation (A).##EQU1## where h: thickness of the steel (mm)

σ_(T) : tensile stress (kg/mm²) in a longitudinal direction of the steelstrip

D: diameter of the cooling roll (mm)

I: a positive coefficient

a, b, and c: positive factors

In addition, if tensile stresses σ_(T) in the longitudinal direction ofthe steel strip 1, winding about the cooling roll 2 are not uniform inthe traverse direction of the steel strip, contact angles θ₂ in parts ofthe steel strip subjected to higher tensile stress are larger than thosein parts of the steel subjected to lower tensile stress. For example,when the tensile forces in the proximity of edges of the steel strip arehigher than those in the center of the steel strip, contact angles θ'₂at the edges are larger than contact angles θ"₂ at the center of thesteel strip as shown in FIG. 6. In the event that contact angles θ₂ aredifferent in the traverse direction of the steel strip, the parts of thesteel having larger contact angles θ₂ will be in contact with thecooling roll for a longer period of time than that of the smallercontact angles θ₂, so that the temperature fall in the former parts ismore than that in the latter parts to provide temperature differences inthe traverse direction of the steel strip. The temperature difference inthe traverse direction is caused in this manner.

Upon denoting the temperature difference in the traverse direction ofthe steel strip by ΔΔT_(S), it is indicated in the following equation(B) with temperature fall ΔT_(S) of the steel strip per one coolingroll, an average contact angle θ among contact angles selected along thetraverse direction and a difference Δθ between the contact angles.##EQU2## where K is a constant.

As can be seen from the equation (B), it is clear that (1) the largerthe temperature fall ΔT_(S) of the steel, the larger is the temperaturedifference ΔΔT_(S) in the traverse direction, (2) the larger thedifference Δθ in contact angle, the larger is the temperature differenceΔΔT_(S) and (3) the smaller the average contact angle θ, the larger isthe temperature difference ΔΔT_(S).

The contact angle difference Δθ corresponds to θ₁ -θ₂ in FIG. 5.Accordingly, the value Δθ is determined by the tensile stress σ_(T) inthe lengthwise direction of the steel strip, the diameter D of thecooling roll and the thickness h of the steel strip as above described.

The buckling of the steel strip is caused by the compressive forces inthe steel due to the temperature difference in the traverse direction ofthe steel as above described. The steel strip is thus likely to causethe buckling in the event of the larger temperature difference ΔΔT_(S)in the traverse direction. Accordingly, a buckling limit of a steelstrip in roll cooling can be considered correspondingly to thetemperature difference ΔΔT_(S) in the traverse direction.

As above described, the factors for determining the temperaturedifference ΔΔT_(S) are the temperature fall ΔT_(S) per one cooling roll,the average contact angle θ and contact angle difference Δθ in thetraverse direction. On the other hand, the factors for determining thecontact angle difference Δθ are the tensile stress σ_(T) in thelengthwise direction of the steel strip, the diameter D of the coolingroll and the thickness h of the steel strip. The temperature differenceΔΔT_(S) is indicated by the following equation (c) by substituting theequation (A) with the relation Δθ=θ₁ -θ₂ into the equation (B). ##EQU3##

If the value ΔΔT_(S) is less than a determined value, no buckling canoccur any longer in the steel strip. If such a determined value isdenoted by J, the condition J>ΔΔT_(S) for avoiding the buckling of thesteel strip is expressed by an equation (D) from (C) ##EQU4##

Now, the diameter D of the cooling roll is limited in a relation##EQU5## so that the equation (D) is simplified as an equation (E).

    ΔT.sub.S <F×σ.sub.T.sup.a ×θ×h.sup.-c (E)

The condition in roll cooling for avoiding the buckling of the steelstrip can be obtained by determining the factors F, a and c. Theinventors determined values of these factors by the followingexperiment.

Experiment I

Steel bands having thicknesses within 0.2≦h<0.6 mm were cooled bycooling rolls having a diameter of 600 mm with tensile stresses 0-4kg/mm². FIG. 7 illustrates a part of results of the experiment, whereinthe steel strips of a thickness h=0.4 mm are subjected to a tensilestress σ_(T) =1 kg/mm² to study values θ, ΔT_(S) and limits ofacceptable cooled steel shapes.

FIG. 11 illustrates relations between the temperature fall ΔT_(S) andthe tensile stress σ_(T) with steel strips of a thickness h=0.4 mm woundabout cooling rolls with winding angles of 30°, 60°, 90° and 120°. Areasbelow the respective straight lines in FIG. 11 are good shape areas.FIG. 12 shows relations between the temperature fall ΔT_(S) andthickness h of steel strips subjected to tensile stress 1 kg/mm² withwinding angles θ. Areas below the respective straight lines are goodshape areas. The factors in the equation (E) were determined by usingthe above results of the experiment to obtain an equation (1).

    ΔT.sub.S <0.65·σ.sub.T.sup.1.5 ·θ·h.sup.-0.75                    (1)

In this case, θ represents "winding" angle, because the differencebetween the contact and winding angles is very small in comparison withthe actual winding angles such as 30°-120° and the actual operationshould be controlled by winding angles instead of theoretical contactangles. The "winding" angle θ is therefore used in substitution for"contact" angle hereinafter and in claim.

Experiment II

Steel bands having thicknesses within 0.6≦h≦2.3 mm were cooled bycooling rolls having a diameter of 1,000 mm with tensile stresses 0-4kg/mm². FIG. 8 illustrates a part of results of the experiment, in whichthe steel strips of a thickness h=1.0 mm are subjected to a tensilestress σ_(T) =1 kg/mm² to study values θ, ΔT_(S) and limits ofacceptable cooled steel shapes.

FIG. 13 illustrates relations between the temperature fall ΔT_(S) andthe tensile stress σ_(T) with steel strips of a thickness h=1.0 mm woundabout cooling roll with winding angles of 30°, 60°, 90° and 120°. Areasbelow the respective straight lines in FIG. 13 are good shape areas.FIG. 12 shows relations between the temperature fall ΔT_(S) andthickness of steel strips subjected to tensile stress 1 kg/mm² withwinding angles. Areas below the respective straight lines are good shapeareas. The factors in the equation (E) were determined by using theabove results of the experiment to obtain an equation (2).

    ΔT.sub.S <1.05·σ.sub.T.sup.1.5 ·θ·h.sup.-0.83                    (2)

With cooling rolls having diameters larger than those used in the aboveexperiments, the range of the temperature fall ΔT_(S) becomes wider ascan be seen from the equation (D). When the temperature fall ΔT_(S) iswithin the ranges of the equations (1) and (2), respectively for thespecified thicknesses of the steel strips and diameters of the coolingrolls, the steel strips can be cooled keeping the steel strips in goodshapes.

FIG. 9 illustrates relations between the tensile stress σ_(T) and theremaining factors ##EQU6## with steel strips of thicknesses 0.2≦h<0.6 mmusing rolls having 1,000 mm diameters showing how the tensile stressaffects the shapes of the cooled steel strips. It representssubstantially the same relation as the equation (1).

FIG. 10 illustrates the relations similar to those in FIG. 9 withexception of the thicknesses 0.6≦h≦2.3 mm of steel strips and diameters1,200 mm of the cooling rolls.

The following conclusion was obtained from the above experiments of rollcooling.

1. When steel strips of thicknesses of 0.2≦h<0.6 mm are treated, theroll cooling operation can be effected without any defective change inshape of the steel strip by fulfilling the condition of the equation (1)

    ΔT.sub.S <0.65·σ.sub.T.sup.1.5 ·σ·h.sup.-0.75,

with cooling rolls of more than 600 mm diameters.

2. When steel strips having thicknesses within 0.6≦h≦2.3 mm are treated,the roll cooling operation can be effected without any defective changein shape of the steel strip by fulfilling the condition of the equation(2)

    ΔT.sub.S <1.05·σ.sub.T.sup.1.5 ·θ·h.sup.-0.83,

with cooling rolls of more than 1,000 mm diameters.

FIGS. 9 and 10 clearly illustrate the relations between principalfactors including thicknesses of steel strips defectively affectingtheir shapes after cooled, so that roll cooling conditions withoutcausing any defective change in shape of the steel strip can easily bedetermined depending upon the thicknesses of the steel strip to becooled.

As can be seen from the above description, according to the inventionsteel strips can be properly cooled with cooling rolls without anydefective deformation of the steels.

It is further understood by those skilled in the art that the foregoingdescription is that of the preferred embodiment of the disclosed methodand that various changes and modifications may be made in the inventionwithout departing from the spirit and scope thereof.

What is claimed is:
 1. A method of cooling a steel strip with a hollowcooling roll by means of thermal transmission through the roll between acooling medium flowing through an internal cavity of said cooling rolland said steel strip being in contact with and running about saidcooling roll, wherein said method comprises the steps of obtaininguniform contact between the strip and cooling rolls, controlling thetemperature fall of the strip per each cooling roll, and controllingtensile forces on the strip, said method being carried out so as tofulfil the following equation (1) with said cooling roll having adiameter D>600 mm when a thickness h of said steel strip is within0.2≦h<0.6 mm,

    ΔT.sub.S <0.65·σ.sub.T.sup.1.5 ·θ·h.sup.-0.75                    ( 2)

and so as to fulfil the following equation (2) with said cooling rollhaving a diameter D>1,000 mm when a thickness h of said steel strip iswithin 0.6 mm ≦h,

    ΔT.sub.S <1.05·σ.sub.T.sup.1.5 ·θ·h.sup.-0.83                    ( 2)

where ΔT_(S) is temperature fall °C. per one cooling roll, σ_(T) istensile stress in a lengthwise direction of said steel strip and θ iswinding angle about said cooling roll.